Optical imaging systems using conventional spherical lenses generally require a large number of surfaces, and hence a large number of elements, in order to correct for optical aberrations present in the system, and thereby to improve the image quality. In principle, if the number of elements is unlimited, the optical system designer can propose spherical lens assemblies which can, in almost every case, simultaneously correct for all of the common optical aberrations in a lens system of any desired f-number. However, the number of surfaces required to do this may be so high that the resulting lens assembly is excessively large in size and weight, and expensive to produce. Furthermore, because of the residual reflections from each surface, and the bulk absorption in each lens, the transmission of the complete lens assembly may be unduly reduced.
The use of aspheric surfaces, with or without the incorporation of diffractive elements, allows the design and construction of lens assemblies with the same or even better optical performance than an equivalent all-spherical system, but in most cases, with a significant reduction in the number of elements required, and therefore a significant improvement in the overall lens assembly size, weight, cost and optical transmission. In many cases, each aspherical surface in an optical system can be used to replace at least two spherical surfaces. This advantage becomes particularly important in the construction of lens systems for use in thermal imaging systems, such as those which operate in the 8 to 12 micron or the 3 to 5 micron wavelength regions. In order to increase the sensitivity of such systems, the lenses used often have large apertures of the order of several inches. Furthermore, of the materials available for use in these spectral regions, such as germanium, CVD-grown zinc selenide or zinc sulphide, silicon, gallium arsenide, calcium fluoride, and others, some are very expensive, and savings engendered by a reduction in the number of elements, both in material costs and in production and coating costs, are therefore a very significant factor in reducing total system cost. These savings usually outweigh the additional cost of production of the aspheric surfaces.
There are three main methods of producing aspheric surfaces on optical lenses. For the production of low precision aspheric optical elements for use in the visible or near infra-red, aspheric elements are made by casting or molding materials such as glass or optical grade polymers. Because the molds are so expensive to manufacture, such lenses have been used in mass produced optical equipment such as still and video cameras, and in optical disc readers, such as video disc players and optical memory discs. Such lenses generally contain aspheric elements with one side aspheric and the other side plane or spherical.
A number of patents have recently been granted for inventions which use mass produced lenses with both surfaces of aspherical form. In what is possibly the earliest such patent, U.S. Pat. No. 4,449,792 to N. Arai, S. Ishiyama and T. Kojima, a large aperture single lens is described having both surfaces aspheric. The lens is designed for use as the pickup lens in a video disk reader, and is made of plastic to make it lightweight. A similar lens has been described by M. Koboyashi, K. Kushida and N. Arai, in U.S. Pat. No. 5,475,537. An optical system with improved performance is described, for use in recording and reading information at visible or near infra-red wavelengths on an optical information medium. The system uses a double-sided aspheric objective lens for the imaging function. This lens is described as being made of glass or of “resin”, the resin presumably being a transparent optical grade plastic material. In U.S. Pat. No. 5,583,698 to K. Yamada et al, is described a double aspheric lens for use in a video camera zoom lens. In U.S. Pat. No. 5,642,229 is described a double-aspheric lens for use in a projection lens unit, while in U.S. Pat. No. 5,726,799, a double aspheric lens is described for use in the viewfinder of a compact camera.
All of the above patents describe double aspheric lenses made of glass or plastic materials, and for use in the visible or near infra-red. These are suitable materials from which lenses can be manufactured at low cost, by casting or by molding. Though it is possible to manufacture a mold which will produce high precision cast or molded double aspheric lenses, the system requirements of such lenses are not usually high enough to warrant the cost of such precision. Molding or casting are therefore used, in general, to provide elements with sufficient precision for the requirements of such comparatively low cost, mass produced systems.
Almost all of the optical elements in optical imaging systems for use in the thermal infra-red region are made of materials which cannot be easily cast or molded, if at all. More important, even if they could be manufactured by these methods, the optical precision in these imaging systems is such that the precision afforded by these methods, at a reasonable manufacturing cost, generally falls far short of the system requirements. Maximum surface peak-to-valley irregularities of the order of λ/2 at the red HeNe wavelengths (0.63 μm) are required in the elements of many such systems to ensure adequate performance. The accuracy of optical surfaces are often measured by using the interference fringes of red HeNe laser light, and for this reason, a description of accuracy in terms of wavelengths of red HeNe laser light is used throughout this specification and is also thus claimed.
The second production method for producing aspheric surfaces are specialized variations of conventional polishing techniques, wherein position dependent pressure is applied to the polishing pad to produce the aspheric form. This method is very labor intensive, and can generally only be used to produce slight asphericity. Recently, automated machines for performing such polishing have been developed.
On an industrial scale, the current almost universally used method of producing the aspheric surfaces of such precision elements, especially those for thermal imaging systems, is by means of turning with a single crystal diamond tool, on a special purpose, ultra-high precision, vibration-free lathe, whose spindle runs at medium to high speeds in air bearings, and which generally uses laser metrology in order to measure the progress of the work. Such diamond turning lathes are capable of accuracies of better than 25 nanometers, and can produce an optical surface of sufficiently high quality for use in the elements of such thermal imaging systems. The cutting tool used is generally a single crystal diamond, specially shaped to provide a smooth cut, though other suitable single point turning tools may also be used. The aspheric profile is obtained by suitable CNC control of the motion of the single point cutting tool relative to the workpiece. Diamond machining can be efficiently applied for small or large production quantities, and for many of the currently used infra-red materials, and others. Furthermore, diamond cutting technology can be used for cutting diffractive patterns in addition to the aspheric surface, thereby further increasing the optical performance of the element.
Single point machining of precision optical elements is also used in a fly cutting configuration, wherein the workpiece is substantially static and the cutting tool is rotated at high speed over the element to provide the machining cut. The desired surface profile is obtained by suitable CNC control of the relative motion between the cutting tool holder and the element being produced. Fly cutting is often used in order to produce precision elements without an axis of rotational symmetry, such as precision cylindrical or elliptical surfaces on mirrors or on transmissive elements. Throughout this patent, the use of the term aspheric is understood also to include such cylindrical or elliptical surface shapes. In addition, flat surfaces are also commonly prepared by means of fly cutting.
Throughout this patent, the use of terms such as “turning”, “machining”, “single point machining”, “fly cutting”, “diamond turning” or “diamond machining” or their equivalents, which may have been used interchangeably, are all understood to mean forms of very high precision surface material removal using a sharp tool to produce an optical quality surface, whether strictly termed turning, machining, milling, or any other similar surface material removal technology, and whether the tool is of diamond or of any other suitable material.
However, up to now, it has been possible to produce elements with only one aspheric surface using diamond turning. The second surface has to be either planar or spherical. This has limited the ultimate usefulness of aspheric surfaces in the art of optical design for the thermal infra-red region, since present optimal designs may still require a larger number of lenses than would be required if lenses with both surfaces aspheric were available.
The reason that precision lenses can currently be produced with only one surface aspheric is an outcome of the methods currently available for holding the workpiece during machining and for testing the holder. The element must be tightly held during machining, yet without imposing any internal stresses. This is done using a vacuum chuck, with the rear surface of the workpiece in intimate contact with the chuck surface. The chuck has a diameter at least as large as that of the element itself, and the vacuum is delivered to the rear surface of the element by means of a series of circular grooves cut in the chuck surface. The use of vacuum chucking, which pulls the rear surface of the workpiece onto the chuck surface, is an important factor in supporting the workpiece without stress. If the workpiece were mechanically clamped at its edge, as in conventional turning or fly cutting, it would be under deformation while being worked, and though perfectly formed while in the chuck, it would spring back on release to its unstressed position, thereby losing its precision form.
In addition, in order to ensure stress free seating of the workpiece in the chuck, the chuck surface itself is made to optical quality. In the words of P. R. Hall in the article entitled “Use of aspheric surfaces in infra-red optical designs” published in Optical Engineering, Vol. 26, pp. 1102–1111 (November 1987), the disclosure of which, and of all documents cited therein, are hereby incorporated by reference, “It is an essential feature of diamond machining that the workpiece is chucked on reference surfaces that have been cut by the machine.” The chuck surface itself is therefore made by diamond turning, if spherical, or, by diamond fly cutting if flat. The rear surface of the workpiece is then given an optical surface of identical mating shape, so that it sits in the chuck stress-free.
The main reason that only flat or spherical chucking surfaces are currently used arises from the fact that even in a well produced batch of aspheric surfaces, each one has slightly different surface irregularities, to a much greater degree than conventionally polished spherical surfaces. Therefore, even if an accurate aspheric chuck were made, each aspherically produced first surface would sit slightly differently in the chuck, and would prevent accurate machining of the second surface.
A secondary reason which has prevented the use of aspheric chucks, arises from the need to test the accuracy of the chuck surface being turned or cut while it is still on the diamond turning or fly-cutting machine. This is necessary so that figure corrections can be made in successive cuts, in order to obtain a highly accurate final cut. This testing is done by looking for the interference fringes between the worked surface and a suitable test glass using monochromatic light. Since aspheric test glasses are considerably more difficult to produce than spherical ones, up to now, only spherical or flat chucks have been generally produced.
There exist other methods of measuring the power and irregularity of an aspheric surface of the chuck, such as with a profile measurement instrument such as the Rank Talysurf, manufactured by Rank Taylor Hobson of Leicester, England. Use of this instrument is not feasible for two reasons. Firstly, the presence of the vacuum grooves in the chuck surface interfere with the measurement, and these grooves must be cut before the surface is finished, as otherwise, cutting the grooves would distort the finished surface. Secondly, the Talysurf can only be used while the surface under test is off the machine, and removal and replacement of the chuck between testing off-line and machining would effectively degrade the accuracy of the whole machining process.
In the above-mentioned article by P. R. Hall (op. cit.), which compares a number of aspherically designed lens systems with equivalent systems designed using only spherical lenses, the author always refers to the addition of one aspherical surface per element, and of a second aspherical surface being added only to a second element.
Similarly, in U.S. Pat. No. 5,668,671 to M. Erdmann, assigned to British Aerospace plc, one of the leaders in the field of diamond turned lens design and production, there is described a dioptric imaging lens system for use in the thermal infra-red region. The lens elements are made of germanium and silicon, presumably with any aspheric surfaces diamond-turned to provide the precision required for the application. The only aspheric lens described or claimed therein, has only one of its surfaces aspheric.
There therefore exists a need for double-sided precision aspheric lenses, with or without the addition of diffractive patterns, especially for use in the thermal infra-red region, for improving the transmission and imaging performance of such optical systems.
The disclosures of all publications and patents mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications and patents, are hereby incorporated by reference.